Activating Octahedral Center in Co-Doped NiFe 2 O 4 via Bridging Amorphous MoS x for Electrocatalytic Water Oxidation: A Case for e g Orbital Regulation in Spinel Oxide.

Moderate e g filling for octahedral metal cations (M Oh ) is strongly correlated with the electrocatalytic water oxidation performance in the oxides system. Here, the e g fillings of Ni Oh and Fe Oh in NiFe 2 O 4 -based spinel are controllably regulated by introducing an external radical of catalytically inactive MoS x as an electron acceptor via a novel ultrasonic anchored pyrolysis strategy. The electron occupied in e g orbit of M Oh emigrates with the amount of MoS hanging on the apical of octahedral sites, and results in a salutary transition from high to medium e g occupancy state, as confirmed by the X-ray absorption spectroscopy and X-ray photoelectron spectroscopy. In addition, benefiting from the abundant unsaturated S atoms in amorphous MoS x , the M Oh at the surface furthest activates and consequently shows a superior water oxidation performance. Density functional theory also reveals that the e g fillings of Ni and Fe decrease to 1.4 and 1.2 after MoS x modification, which can effectively reduce the free energy of the OOH* intermediates in the oxygen evolution reaction process. This work opens an avenue for further releasing the electrocatalytic activity of octahedral sites through bridging external phases with rational electron-capturing/donating capability.